The development of computer hard disk drives demands ever increasingly higher track density, lower acoustic noise, and better reliability under shock and vibrational disturbances. The undesirable characteristics of the widely used ball bearing spindles, such as high non-repetitive runout, large acoustic noise, and high resonance frequencies due to bearing defect, impose severe limitation on the drive's capacity and performance.
The use of a non-contact bearing, such as a hydrodynamic bearing ("HDB"), may overcome the aforementioned limitation. The full film lubrication of a fluid bearing displays significantly lower non-repetitive runout and acoustic noise, and its higher damping provides better resistance to external shock and vibration.
The deployment of the HDB system in a hard disk drive environment requires that the lubricant be securely sealed inside of the bearing structure under all operating and non-operating conditions in order to prevent performance degradation of the bearing and contamination in the drive. At the same time, the bearing system needs to have enhanced dynamic performance and be easily manufacturable in order to satisfy cost requirements. As explained below these requirements often come into conflict with each other and have heretofore resulted in compromised HDB spindle designs.
There have been a number of prior approaches for providing seals for hydrodynamic bearing units. Static seals, such as O-rings and surface tension or capillary seals, have been employed to seal hydrodynamic bearings.
An example of capillary seals is disclosed in commonly-assigned U.S. Pat. No. 5,423,612 entitled: "Hydrodynamic Bearing and Seal", the disclosure thereof being incorporated herein by reference. One drawback of the approach described in the '612 patent is it has proven somewhat difficult to provide recirculation ports around the bearings in order to realize a lubricant recirculation capability in circumstances such as imbalanced pumping and/or shock load. Another drawback is that since both top and bottom seals are at the inside diameter of the HDB unit, any splashed droplets which separated from the lubricant surface may be driven out of the bearing by centrifugal force. In addition, because of the small seal volume available at the HDB unit inside diameter, the lubricant may leak out of the bearing due to thermal expansion and/or filling volume variations. Finally, another drawback is that the capillary seals are defined such that a larger taper angle was not manufacturably possible. That is, to increase the taper angle of the seals, shaft diameter at the ends would have to be decreased or the overall diameter would have to be increased to the point of degrading bearing performance.
Small (3.5 inch disk diameter and smaller) form factor disk drives are used in unlimited applications and orientations. Consequently, a hydrodynamic bearing system for a disk spindle in such drives, e.g. having a full Z-dimension of 1.6 inch height spindle manifesting high inertial loading, must also operate in all possible orientations, and to be able to withstand and sustain certain shock events and vibration levels without leakage. A cover-secured or top-fixed HDB motor is required for disk drives with high inertial load, such as disk drives including six or more rotating disks. For top-fixed spindles, the requirement for two lubricant seals poses a considerable challenge.
Generally, there are two types of top-fixed HDB spindle designs, namely, a single thrust-plate design, and a double thrust-plate design, shown in FIG. 1.
The single thrust plate design described is U.S. Pat. No. 5,423,612 exhibits the drawbacks discussed above.
The double thrust plate design of FIG. 1 was an improvement over the single thrust plate design in some aspects. However, the double thrust-plate design illustrated in FIG. 1 also exhibited the drawback of controlling the tolerance of the total length of the sleeve which defines the thrust bearing clearance (which is about 10 microns). Another drawback related to manufacturing difficulty in controlling the tolerances of perpendicularity and surface finish at both ends of the e.g. bronze sleeve. In addition, because the sleeve is typically made of bronze, the sleeve tends to wear by coming into contact with the grooved steel thrust plate having pumping grooves during starting and stopping intervals.
An HDB improving upon both the single thrust plate design of the '612 patent and the double thrust plate design of FIG. 1 was proposed in commonly assigned U.S. patent application Ser. No. 08/519,842, shown in FIG. 2, the disclosure thereof being incorporated herein by reference. However, the single thrust plate HDB of FIG. 2 still manifests certain drawbacks. Specifically, the thrust bearing clearance may vary due to possible deformation caused by press fitting thrust bushing 70 onto outer sleeve 16 over a short shoulder on sleeve 16 whose length is limited by the location of the top seal. In addition, the diameter of the top seal plate 80 is larger than that of the bottom seal plate 81, which may cause unidirectional pumping by centrifugal force. Another drawback is that the orientation of the capillary seals 91 does not utilize centrifugal force in sealing the lubricant. Bearing lubricant is pulled back only by static forces, i.e. capillary forces. Finally, the capillary seals are oriented such that a requirement for long capillary seal length would reduce the journal bearing length and/or journal bearing span in such a way that dynamic performance is degraded in small form factor spindles e.g. a 1 inch height disk drive.
Thus, a hitherto unsolved need has remained for a hydrodynamic bearing system which is leak free irrespective of orientation, shock and vibration, and which is readily and reliably manufacturable at reasonably low manufacturing cost.